3.1 Stress-strain curves
Figure 1 illustrates the stress-strain curves of thermal treatment
granite under triaxial cyclic loading compression, it is in good
agreement with the stress-strain curves of granite under monotonic
loading except for granite specimen when T = 600°C,σ 3 = 0 MPa. It can be explained that cyclic
loading has a slight effect on the crack propagation of intact granite.
However, when more thermal cracks were induced in rock, the cracks were
easier to propagate under uniaxial cyclic loading, and make the strength
of specimen decreases. Confining pressure can restrain the crack
propagation, therefore cyclic loading has a slight effect on the
stress-strain curves of granite specimen when T = 600°C,σ 3 = 40 MPa.
At initial loading at each cycle, the axial loading and unloading
stress-stain curves show a concave type. It means that fissures in
granite opened under unloading process, and then reclosed under loading
process. After the first loading-unloading compression, the concave
characteristic is not evident, which indicates that the closed fissures
cannot open completely. The concave type is more obvious after the peak
strength, which indicates that the fissures induced by axial stress also
opened and reclosed in the cyclic loading-unloading process. The loading
enter to the elastic stage with increasing axial stress, even though the
macro-crack has been formed, there is also elastic stage after the peak
strength. The loading enter to the plastic stage after the elastic
stage, the plastic stage is unobvious before the peak strength, and it
is more obvious with increasing cycle number after the peak strength.
After the peak strength, the axial stress decreases gradually with cycle
number. However, the residual strength of specimen under the cyclic
loading is larger than that under monotonic loading on the whole.